Apex predators such as Northern elephant seals accumulate toxic methylmercury from their prey in the North Pacific. (Photo credit: Dr. James Harvey)

We’re taught from a young age to be afraid of mercury poisoning — it’s the reason most people are careful never to break a thermometer. But human pollution is putting mercury into the environment anyway, in both the gases we pour into the atmosphere and the waste we dump into our waterways, and one of the biggest places it’s showing up is in the ocean.

That’s bad enough, but to make matters worse, a new study, published Monday in Proceedings of the National Academy of Sciences, suggests that a particularly toxic form of mercury is being shuttled from the open ocean into coastal areas in an unexpected way: in the fur of seals and sea lions.

Mercury gets into the seawater when it’s absorbed from polluted air, or when it’s dumped there by industrial sources along with other pollutants. Mercury is found in many different chemical compounds, most of which can be harmful if they make it into the body — but it can be especially scary when it hits the ocean because of the way it interacts with certain marine organisms.

Tiny microbes in the seawater can convert mercury into a different form, called “methylmercury,” a powerful neurotoxin capable of causing damage to the brain. When an animal eats one of these microbes, the methylmercury is transferred into its body — and it’s capable of moving from one predator to the next, all the way up the food chain, until it reaches humans. This is why it’s important for scientists to keep an eye out for methylmercury “hot spots” in the ocean, which could yield contaminated seafood. Methylmercury poisoning is thought to be especially dangerous for pregnant women and young children.

When methylmercury accumulates in high concentrations along the shore, it’s usually because an industrial pollution source is nearby. But the troublingnew research suggests that methylmercury may be building up in some coastal areas for a different reason: It’s being carried in from the open ocean by contaminated seals, who shed it into the water through their fur.

It may sound unlikely, but scientists have suspected this was happening as far back as the 1970s, when a California research project found unusually high levels of mercury in mussels at the Año Nuevo State Reserve, a popular breeding ground for several species of seals and sea lions.

“At the time, [they] speculated that all the biological material coming from those animals was basically pulsing mercury into the system,” says Jennifer Cossaboon, a master’s student in environmental science at San Diego State University and lead author of the study. But at that time, scientists didn’t have sensitive enough technology to test their theory.

Now, decades later, Cossaboon and a group of colleagues have returned to Año Nuevo to test mercury levels in both the seawater and the fur of elephant seals who reside in the area. They found that methylmercury levels in the seawater at Año Nuevo were significantly higher than those at other sites along the California coast — in fact, about eight times higher, on average — suggesting that the animals had something to do with it.

Methylmercury levels were comparatively high at Año Nuevo all year round, the research showed — but interestingly, there was a definite spike in the amount of methylmercury present during the elephant seal molting season, or the time of the year during which they shed their fur. This makes sense because one of the main ways mercury tends to work its way out of an animal’s body is through its fur, Cossaboon says.

The researchers also tested the molted elephant seal fur at Año Nuevo, just to be sure, and found that mercury was present. Elephant seals spend half their time feeding in the open ocean and half their time on the coast, where they don’t eat at all, Cossaboon says. So the researchers suspect that the seals are picking up mercury by eating contaminated fish offshore, and then bringing that mercury to the coast.

While they only tested elephant seal fur, plenty of previous research has showed that mercury can show up in the fur of other seal and sea lion species too, meaning elephant seals are probably not the only culprits at Año Nuevo. And other types of wildlife, such as seabirds, can also carry mercury in their feathers, so Cossaboon says future research might examine the different animals that could be contributing to mercury contamination along the coast.

However, there’s an important limitation to the research: Scientists still aren’t sure exactly how easily the methylmercury can be taken up by marine organisms as it moves from seal fur into the water. Since they only tested methylmercury levels in the seawater, and not in the shellfish and other small organisms living along the coast, they can’t say for sure yet how much of a problem the mercury is causing to wildlife in the area.

So far, there have been no visible signs that marine organisms are suffering at Año Nuevo, Cossaboon says. And the area is also part of a marine reserve, meaning it isn’t used for fishing and is probably not a direct threat to human health. But Cossaboon believes the issue is still worth further examination.

“Marine mammals and other [animals near the top of the food chain] are commonly ‘sentinel species’ for environmental health,” Cossaboon says, meaning they can signal larger problems within their ecosystems. This study highlights the mercury problem in particular, but also points to the growing need to reduce industrial pollutants, which may be affecting the environment in ways we still don’t understand.

And Año Nuevo is also hardly the only place where seals and sea lions congregate. According to the paper, the U.S. Pacific Coast hosts hundreds of thousands of seals and sea lions, all of which have the potential to affect mercury cycling along the shoreline.

“I think it’s important when trying to understand the global mercury cycle, that we’re really looking at the whole picture,” Cossaboon says. “It’s really interesting to see how wildlife can be exacerbating the cycle.”

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